With 39 million people estimated to be infected with HIV-1 worldwide, the HIV epidemic has become a serious global health problem. Given that 40% of infected individuals live in developing countries where antiretroviral medications are not widely available, the need to develop an effective vaccine is of paramount importance. Our long-term goal is to elucidate viral evolution and immune correlates of protection against infection as necessary prerequisites to the development of an effecitve HIV vaccine. Cytotoxic T lymphocytes (CTL) play a central role in controlling human immunodeficiency virus (HIV) infection in humans and simian immunodeficiency virus (SIV) in rhesus monkeys. However, the high mutation rate of HIV/SIV allows for the constant generation of viral variants that maintain optimal fitness in the context of an evolving adaptive immune response. Viral variants that are capable of subverting potent CTL responses have been shown to result in increased virus replication and disease progression. Therefore, it is important to define the strategies used by virus isolates to mutate away from epitope-specific CTL responses, so that we can anticipate variations in key epitopes and formulate vaccine approaches to avert selection of escape variants. Our preliminary work showed that the appearance of a position 2 p11C, C-M Gag epitope substitution in a simian-human immunodeficiency virus (SHIV) strain 89.6P that allowed viral escape from CTL recognition in an infected Mamu-A*01+ rhesus monkey is temporally correlated with the emergence of a flanking downstream amino acid substitution in the capsid protein. Our hypothesis is that the SHIV-89.6P escape at the highly conserved, immunodominant Gag p11C CTL epitope is infrequent because additional compensatory mutations are required to facilitate this viral escape. In the proposed studies, we will examine the biochemical, structural, and functional relevance of the epitope and flanking mutations on viral core formation using electron microscopy and x-ray crystallization. In addition, we will evaluate the pathogenicity and immunological consequences of the escape variant SHIV-89.6P in vivo. Ultimately, these studies will facilitate the design of novel vaccination strategies that may prevent the evolution of viral escape mutations.